This invention relates to sewing machines having a mechanism permitting a skip stitch mode of operation. Such mechanisms have the capability of interrupting the reciprocating movement of the needle bar for a predetermined number of rotations of the main armshaft while the work feeding mechanism continues to operate. This mode of operation facilitates the sewing of repetitive but contiguous patterns including basting stitches. When interrupting the reciprocating movement of the needle bar, the needle bar drive link which is driven by the main arm shaft is decoupled from the needle bar which is then stored in a substantially full up position. This allows the needle bar drive link to continue to reciprocate and the work feeding mechanism to continue to operate until the needle bar drive link is again coupled to the needle bar for restoring reciprocatory motion thereto. The latching device that effects the coupling and decoupling of the needle bar drive link must do so in an extremely short time. Most such latching devices, when operated at the high speeds that are common in most modern household sewing machines, will occasionally fail to latch. This will cause the machine to skip stitch longer than it should, or to sew during part of the skip stitch cycle resulting in a non-uniform and aesthetically inferior work piece. The common solution to this problem is for the operator to activate a manually operable switch that selects a slow operating speed for the machine. However, should the operator fail to do so, the error may go unnoticed until significant damage is done to the workpiece.
The following cited patents, each deal in some way with automatic speed control of the sewing machine. U.S. Pat. No. 3,780,681, Dec. 25, 1973 by Sasaki discloses a sewing machine including a basting arrangement whereby the arm shaft is driven one revolution at a fixed slow speed independently of the amount of depression of the speed controller. U.S. Pat. No. 3,913,507, Oct. 21, 1975 by Pollmeier discloses a sewing machine having a workpiece cover element which is automatically displaced into an inoperative position when the machine is operated at low speed for precision stitching. Japanese Lay-Open 52-54551, May 4, 1977 by Watanabe et al discloses a sewing machine having a data store which holds the information for each pattern to be sewn. This information includes the maximum rotary speed permissible for each pattern.
The above citations comprise what the Applicant believes to be the closest disclosures that are relevant to the examination of this application.